To add to that: backup generators are also a logistical mess. Diesel doesn't last infinitely. Do in particular if you expect to run large generators to sustain significant load (like in big data centers, I don't know how significant the load is for the backup gens of a nuclear plant), the amount of diesel you get to store is impressive. Which in turn means that the logistics/tank life issues of the fuel get worse.

The physical&power infrastructure side of big data centers is really quite interesting, by the way. Not something we all get exposed to regularly but I can only encourage anyone who has a chance to learn about it to take that opportunity.